To reproduce: please refer to the attached gif file

Dim a1 As New Appointment(DateTime.Today.AddDays(-1), TimeSpan.FromDays(3), "Termin1")
a1.AllDay = True
Dim a2 As New Appointment(DateTime.Today.AddDays(-1), TimeSpan.FromDays(1), "Termin2")
a2.AllDay = True
Me.RadScheduler1.Appointments.Add(a1)
Me.RadScheduler1.Appointments.Add(a2)

Workaround: specify precisely the end date including the time part as well

Dim a1 As New Appointment(DateTime.Today.AddDays(-1), TimeSpan.FromDays(3).Add(TimeSpan.FromMinutes(1)), "Termin1")
a1.AllDay = False
Dim a2 As New Appointment(DateTime.Today.AddDays(-1), TimeSpan.FromDays(2).Add(TimeSpan.FromMinutes(1)), "Termin2")
a2.AllDay = False
 
Me.RadScheduler1.Appointments.Add(a1)
Me.RadScheduler1.Appointments.Add(a2)